Light emitting diodes (LEDs) are inexpensive and widely used as light sources. Their diverse applications include numeric displays, flashlights, liquid crystal backlights, vehicle brake lights, traffic signals, backlights, and the ubiquitous power-on indicator light on almost every electronic device, and modern electrical appliance.
Because LEDs are most often used as light emitters, it is easy to forget that they can also operate as photodiodes, i.e., light detectors. Although most LEDs are designed as light emitters, and not light detectors, all LEDs can effectively operate in either mode.
This interchangeability between solid-state light emission and light detection was first described in the 1970's, but has since been largely forgotten by LED users, see Mims, “Siliconnections: Coming of Age in the Electronic Era,” McGraw-Hill, New York, N.Y., 1986, and Mims, “LED Circuits and Projects,” Howard W. Sams and Co., Inc., New York, N.Y., 1973.
Light emitting diodes emit light in a fairly narrow frequency band when a small current is applied in the correct direction through the diode, i.e., with a forward bias. Because the current-voltage characteristic is exponential, it is difficult to control a voltage applied directly across an LED accurately enough to attain a desired current.
Therefore, some means must be provided to limit the current. In discrete electronic systems, this is typically done by placing a resistor in series with the LED. Because most microprocessor I/O pins can sink more current than they can source, the configuration shown in the FIG. 1 is the most common way of driving an LED from a microprocessor or microcontroller.
FIG. 1 shows a typical prior art LED emitter circuit 100. An I/O pin 101 of a microprocessor 100 is used to sink current through an LED 102 with a resistor 103 to limit the amount of current.
One important application that uses LEDs is optical signal communications. In most prior art optical communications applications, an LEDs is used in the transmitter, and a photodiode is used in the receiver. In addition, Each component is typically driven separately by a specially designed circuit. The photodiodes are most often specifically designed to receive optical signals in a specific narrow frequency range. Most photodiodes cannot emit light. Consequently, there is one circuit that drives the transmitter, and another circuit for driving the receiver. This increases the cost and complexity of the communications system.
Therefore, it is desired to provide a light emitting diode that can be used as both a transmitter and receiver in an optical communications system.